PSI - Issue 42

P. Ferro et al. / Procedia Structural Integrity 42 (2022) 259–269

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P. Ferro et al. / Structural Integrity Procedia 00 (2022) 000 – 000

challenges linked to sustainability, such as the improvement of engines thermal efficiency or mitigation of the supply risk and economical importance of critical raw materials (CRMs) identified by the European Commission, Nickel based alloys and their joining techniques are certainly worthy of investigations. In fact, in recent works (Ferro and Bonollo 2019; Ferro et al., 2020) dealing with design for recycling and substitution in a critical raw materials perspective, it was shown how nickel-based alloys are excellent candidates for application undergoing severe environmental conditions. Generally speaking, nickel superalloys, such as Inconel 625 (IN625), present at high temperature an exceptional combination of high mechanical strength and excellent corrosion resistance (Reed, 2006). A recent review of the status of technology in design and manufacture of wrought polycrystalline Ni-base superalloys for critical engineering applications can be found in (Hardy et al., 2020). Ni-base superalloys currently represent more than 50% of the weight of advanced aeronautical turbines and are also important for applications in energy production, naval propulsion, extraction of oil and gas, spacecraft, nuclear reactors, heat exchangers (Pollock and Sammy, 2006) and hydrogen technology. Even though Ni-based superalloys cost from 3 to 5 times the iron-based ones, their use is expanding especially in gas turbine components to produce energy because higher temperatures of the thermal cycle guarantee greater efficiency and reduction of polluting emissions. The demand of Ni-based superalloys is expected to expand also for the energy production through conventional steam turbine plants for achieving super critical conditions with a predicted increase of efficiency to 60% and reduction of CO 2 to about 0.7 ton/kWatth while actual sub-critical power plants have an efficiency of 35% and produce 1.2 ton/kWatth of CO 2 . However, higher operating temperatures involve more severe degradation of the mechanical components due to several factors: i) microstructure evolution including formation of undesired phases, coalescence of  ’ precipitates, degeneration of car bides due to fatigue and creep exposure; ii) the formation of cracks. In the first case, heat treatments or Hot Isostatic Pressing (HIP) are used to restore the original microstructure as much as possible; in the second case, joining techniques are used, which could give rise to defects or imperfections, such as discontinuities, local segregations, inadequate penetration, resulting in poor joint resistance (David et al., 1997). Furthermore, cracks may form also in the production process of components made of Ni-based superalloys for energy applications such as flanges, valve bodies, and headers (Viswanathan et al., 2005; deBarbadillo et al., 2016). According to Andersson (2018), Ni-alloy welding issues can be broken down into two broad classifications: geometrical and metallurgical. The geometrical issues include the shape of the weld pool (tear drop tending to be more crack prone), location of the weld (concave tending to be more crack prone), as well as residual stress (RS) and weld defects [13]. The metallurgical issues mainly include strain age cracking (SAC) (Berry and Hughes, 1969; Franklin and Savage, 1974; Thompson et al., 1968) and hot cracking (DuPont et al., 2009; Cieslak et al., 1988; Cieslak et al., 1990). Welding of Ni-base superalloys has gained increasing importance in the repair of mechanical components for the first stages of turbines. The microstructure in the welded metal (WM) of the joint forming during solidification depends on two phenomena: the formation of dendrites and the partition of the solute with consequent possible formation of carbides, borides and different intermetallic phases. Some of these low-melting compounds can trigger micro-cracks during PWHT (Ojo et al., 2004; Henderson et al., 2013). Furthermore, during the solidification localized stresses can develop in the welded area causing mechanical failures (Jensen, 2002). Prediction of such localized stresses is not an easy task but extremely important to correctly design a welding process. In this regard, finite element method (FEM) is certainly a powerful tool provided that the main phenomena inducing residual stresses are considered and correctly modelled. However, for design reasons, the models should not be time consuming. When residual stresses (RSs) are the goal of the simulation, computational welding mechanics (CWM) could be a reliable numerical strategy (Ferro et al., 2005). As a matter of fact, the fusion zone (FZ) is modelled by using proper power density distribution functions reproducing the volume and shape of the molten pool without solving the complex equations of fluid dynamics (Ferro et al., 2010). Moreover, solid-state phase transformations effects on RS can be easily considered using alloy metallurgical constitutive equations (Ferro, 2012; Ferro and Petrone, 2009). Unfortunately, to the best of the authors knowledge, very few works dealing with welding simulation of IN165 can be found in literature. Thejasree et al. (2021) proposed a thermal model of the laser beam welding of IN625 basing on CWM and Sysweld® numerical code. Even if thermal results were found in good agreement with experiments, no mechanical model was implemented for RS prediction. A numerical model of IN625 TIG welding was also developed by Siwek et al. (2013), who implemented an adaptive mesh approach to speed up the simulation process. However, again authors gave up assessing RSs. Finally,